Instrumentation & Methods:
Gamma Spectroscopy
Lynn West
Wisconsin State Lab of Hygiene
Instrumentation –
Gamma Spectroscopy/Alpha Spectroscopy
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Quick review of Radioactive Decay (as it relates to
σ & γ spectroscopy)
Interaction of Gamma Rays with matter
Basic electronics
Configurations
Semi-conductors
Resolution
Spectroscopy
Calibration/Efficiency
Coincidence summing
Sample Preparation
Daily instrument checks
Review of Radioactive Modes of Decay
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Properties of Alpha Decay
z
z
z
Progeny loses of 4 AMU.
Progeny loses 2 nuclear charges
Often followed by emission of gamma
226
88 Ra
222
Rn
86
+ 42He + energy
1
Review of Radioactive Modes of
Decay, Cont.
Properties of
Alpha Decay
z
z
Alpha particle and
progeny (recoil
nucleus) have welldefined energies
spectroscopy based
on alpha-particle
energies is possible
Counts
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4.5
5.5
Energy (MeV)
Alpha spectrum at the theoretical
limit of energy resolution
Review of Radioactive Modes of
Decay, Cont.
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Properties of beta (negatron)
decay
z
z
z
z
z
No change in mass number of progeny.
Progeny gains 1 nuclear charge
Beta particle, antineutrino, and recoil
nucleus have a continuous range of
energies
no spectroscopy of elements is possible
Often followed by emission of gamma
Review of Radioactive Modes of
Decay, cont.
Counts
Cl-36
Ar-36
Energy (MeV)
Beta Emission from Cl-36.
From G. F. Knoll,
Radiation Detection and Measurement, 3rd Ed., (2000).
2
Review of Radioactive Modes of
Decay, Cont.
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Properties of Positron decay
z
z
z
z
No change in mass number of progeny
Progeny loses 1 nuclear charge
Positron, neutrino, and recoil nucleus
have a continuous range of energies
no spectroscopy of elements is possible
Positron is an anti-particle of an
electron
Review of Radioactive Modes of
Decay, Cont.
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Properties of Positron decay
z
z
z
When the positron comes in contact
with an electron, the particles are
annihilated
Two photons are created each with an
energy of 511 keV (the rest mass of an
electron)
The annihilation peak is a typical
feature of a spectrum
Review of Radioactive Modes of
Decay, Cont.
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Other modes of decay
z
Electron Capture
Neutron deficient isotopes
Electron is captured by the nucleus from
an outer electron shell
{ Vacancy left from captured electron is
filled in by electrons from higher energy
shells
{ X-rays are released in the process
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3
Review of Radioactive Modes of
Decay, Cont.
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Other modes of decay
z
z
z
Auger electrons
{ Excitation of the atom resulting in the ejection
of an outer electron
Internal conversion electrons
{ Excitation of the nucleus resulting in the
ejection of an outer electron
Bremsstrahlung
{ “Braking” radiation
{ Photon emitted by a charged particle as it
slows down
{ Adds to the continuum
Review of Radioactive Modes of
Decay, Cont.
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Gamma Emission
z
z
z
No change in mass, protons, or
neutrons
Excess excitation energy is given off as
electromagnetic radiation, usually
following alpha or beta decay
Gamma emissions are high-energy,
short-wave-length
Source:
http://lasp.colorado.edu
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Review of Radioactive Modes of
Decay, Cont.
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Gamma Emission Decay Schemes
KEY
PE Photoelectric absorption
CS Compton scattering
PP Pair production
γ gamma-ray
e- Electron
e+ Positron
γ
Source
γ
e-
γ
γ
γ
e+
511
CS
e-
γ
e-
CS
PP
ee+
CS
511
511
γ
γ
Pb Shielding
Pb Shielding
ee-
γ
PE
ePb X Ray
γ
511
γ
Gamma Spectrum Features
Source: Practical Gamma-Ray Spectrometry, Gilmore & Hemingway
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Resolution
Basic Electronic Schematic – Gamma
Spectroscopy
Low Voltage
Supply
Detector
Preamplifier
Multichannel
Analyzer (MCA)
Amplifier
Detector Bias
Supply
Configurations of Ge Detectors
Electrical contact
True coaxial
Closed-end coaxial
n+ contact
Holes
Electrons
Holes
Electrons
+
p+ contact
p-type coaxial,
∏-type
n-type coaxial,
v-type
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Nature of Semi-conductors
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Good conductors are atoms with
less than four valence electrons
atoms with only 1 valance electron
are the best conductors
examples
z
z
z
copper
silver
gold
Nature of Semi-conductors, Cont.
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Good insulators are atoms with
more than four valence electrons
atoms with 8 valance electron are
the best insulators
examples
z
noble gases
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Nature of Semi-conductors, Cont.
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Semiconductors are made of atoms
with four valence electrons
they are neither good conductors
nor good insulators
examples
z
z
germanium
silicon
Nature of Semi-conductors, Cont.
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Energy Band Diagram
CONDUCTION
BAND
FORBIDDEN
BAND
CONDUCTION
BAND
CONDUCTION
BAND
FORBIDDEN
BAND
VALENCE BAND
VALENCE BAND
VALENCE BAND
Insulator
Semiconductor
Conductor
Nature of Semi-conductors, Cont.
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Covalent bonds are formed in
semiconductors
z
z
z
the atoms are arranged in definite
crystalline structure
the arrangement is repeated
throughout the material
each atom is covalently bonded to 4
other atoms
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Nature of Semi-conductors, cont.
Pure Semi-conductor
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Each atom has 8 shared electrons
there are no free electrons
z
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or no electrons in the conduction band
however, thermal energy can cause
some valence electrons to gain
enough energy to move in to the
conduction band
z
this leads to the formation of a “hole”
Nature of Semi-conductors, cont.
Pure Semi-conductor
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Both holes (+) & free electrons (-)
are current carriers
a pure semi conductor has few
carriers of either type
more carriers lead to more current
doping is the process used to
increase the number of carriers in a
semiconductor
Nature of Semi-conductors, cont.
Pure Semi-conductor
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Impurities can be added during the
production of the semiconductor,
this is called doping
The impurities are either trivalent or
pentavalent
trivalent examples
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pentavalent examples
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z
z
indium, gallium, boron
arsenic, phosphorus, antimony
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n-type Semiconductor
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An impurity with 5 valence electrons
(group V) will form 4 covalent
bonds with the atoms of the
semiconductor
One electron is left over & loosely
held by the atom
This type of impurity is known as
donor impurities.
There are more negative carriers
n-type Semiconductor
CONDUCTION
BAND
Donor electron
forbidden band
Donor electron
Energy level
Valence electron
forbidden band
VALENCE BAND
p-type semiconductors
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An impurity with 3 valence electrons
(group III) will form 3 covalent
bonds with the atoms of the
semiconductor
The absence of the fourth electron
leaves a hole
This type of impurity is known as
acceptor impurities.
There are more positive carriers
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p-type Semiconductor, cont.
CONDUCTION
BAND
Acceptor hole
forbidden band
Acceptor hole
Energy level
Valence electron
forbidden band
VALENCE BAND
Depletion Zone
p-type
+
+
+ ++
++
+
++
+
+
- ++ + +
+
+
++
+ ++
n-type
- -- - -- --- -- +
- ---
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In the depletion zone
the charge carriers
have canceled each
other out
voltage is developed
across the depletion
zone due to the charge
separation
V
Vc
Depletion zone
Calibration/Efficiency
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Ideally, calibration sources would be
prepared such that a point
calibration is performed for each
nuclide reported
z
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this is totally impractical for analyzing
routine unknown samples
Sources should be prepared to have
identical shape and density as the
sample
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Calibration/Efficiency
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Differences in density are less
important than differences in
geometry
z
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Newer software packages allow the
user to create different efficiencies
mathematically
Source strength should not be so
great as to cause pile-up
Calibration/Efficiency
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The calibration energies should cover the
entire range of interest
For close to the detector geometries,
choose a multi-lined source made from a
combination of nuclides which do not
suffer from True Coincidence Summing
(TCS). See Table 7.2 pg 153 Gilmore, G.
and Hemingway, J. 1995. Practical
Gamma-Ray Spectrometry. John Wiley &
Sons, New York
Coincidence Summing
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True Coincidence Summing (TCS)
z
z
z
The summing of gamma rays emitted
almost simultaneously from the nucleus
resulting in a negative bias from the
true value
Larger detectors suffer more from TCS
than do smaller detectors
TCS can be expected whenever
samples contain nuclides with
complicated decay schemes
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Coincidence Summing
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True Coincidence Summing (TCS)
z
z
z
TCS can be expected whenever
samples contain nuclides with
complicated decay schemes
The degree of TCS is not dependent on
count rate
TCS is geometry dependent and is
worse for close to the detector
geometries
Coincidence Summing
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True Coincidence Summing (TCS)
z
z
z
z
TCS is geometry dependent and is
worse for close to the detector
geometries
Summed pulses will not be rejected by
the pile-up rejection circuitry because
the pulses will not be misshapen
For detectors with thin windows X-rays
that would normally be absorbed in the
end cap may contribute to TCS
Well detectors suffer the worst from
TCS
Coincidence Summing
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True Coincidence Summing (TCS)
z
Newer software packages have systems
for reduces this problem
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Coincidence Summing
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Random Coincidence Summing
z
z
z
z
z
Also known as pile-up
Two or more gamma rays being
detected at nearly the same time
Counts are lost from the full-energy
peaks in the spectrum
Affected by count rate
Pile-up rejection circuitry reduces
problem
Sample Preparation
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Acidify water samples
z
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z
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Note: Iodine is volatile in acidic solutions
Active material should be distributed
evenly throughout the geometry
Samples should be homogenous
Calibration materials should simulate
samples (actual or mathematical)
Daily Instrument Checks
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Short background count
Linearity check
Resolution check
Additionally, a long background cout
is needed for backgound subtraction
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Instrumentation & Methods:
Gamma Emitting
Radionuclides USEPA 901.1
Jeff Brenner
Minnesota Department of Health
EPA Method 901.1
Gamma Emitting Radionuclides
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Gamma Emitting Radionuclides
γ
EPA Method 901.1
What we’ll cover
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Scope of the method
Summary of the method
Calibration
z
z
z
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Determining energy calibration
Determining efficiency calibration
Determining system background
Quality control
Interferences
Application
Calculations
z
Activity
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EPA Method 901.1
Scope
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The method is applicable for
analyzing water samples
Measurement of gamma photons
emitted from radionuclides without
separating them from the sample
matrix.
Radionuclides emitting gamma
photons with the following energy
range of 60 to 2000 keV.
EPA Method 901.1
Gamma Emitting Radionuclides Summary
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Water sample is
preserved in the
field or lab with
nitric acid
Homogeneous
aliquot of the
preserved sample
is measured in a
calibrated
geometry.
EPA Method 901.1
Gamma Emitting Radionuclides Summary
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Sample aliquots are counted long
enough to meet the required sensitivity.
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EPA Method 901.1
Gamma Emitting Radionuclides Summary
EPA Method 901.1
Gamma Emitting Radionuclides Summary
EPA Method 901.1 Calibrations
Gamma Emitting Radionuclides
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Library of radionuclide gamma energy
spectra is prepared
Use known radionuclide concentrations in
standard sample geometries to establish
energy calibration.
Single solution containing a mixture of
fission products emitting
z
z
z
z
Low energy
Medium energy
High energy
Example (Sb-125, Eu154, and Eu-155)
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EPA Method 901.1
Gamma Emitting Radionuclides Summary
86.54
105.31
123.07
176.33
247.93
427.89
463.38
591.76
600.56
635.90
692.42
723.30
756.86
873.20
996.30
1004.76
1274.51
1596.45
Eu-155
Eu-155
Eu-154
Sb-125
Eu-154
Sb-125
Sb-125
Eu-154
Sb-125
Sb-125
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
EPA Method 901.1
Gamma Emitting Radionuclides
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Counting efficiencies for the various
gamma energies are determined
from the activity counts of those
known standard values.
A counting efficiency vs. gamma
energy curve is determined for each
container geometry and for each
detector.
EPA Method 901.1
Gamma Emitting Radionuclides Summary
86.54
105.31
176.33
427.89
463.38
600.56
996.30
1004.76
1274.51
Eu-155
Eu-155
Sb-125
Sb-125
Sb-125
Sb-125
Eu-154
Eu-154
Eu-154
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EPA Method 901.1 Calibrations
Gamma Emitting Radionuclides
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FWHM used to monitor peak shape
z
z
{
Smaller tolerance for low energy
Greater tolerance for high energy
Document a few FWHM to
determine instrument drift
EPA Method 901.1
Gamma Emitting Radionuclides Summary
86.54
105.31
123.07
176.33
247.93
427.89
463.38
591.76
600.56
635.90
692.42
723.30
756.86
873.20
996.30
1004.76
1274.51
1596.45
Eu-155
Eu-155
Eu-154
Sb-125
Eu-154
Sb-125
Sb-125
Eu-154
Sb-125
Sb-125
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
Eu-154
EPA Method 901.1
Gamma Emitting Radionuclides Summary
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EPA Method 901.1
(Determine System Background)
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Contribution of the background
must be measured
Measure under the same conditions,
counting mode, as the samples
Background determination is
performed every time the liquid
nitrogen is filled
EPA Method 901.1
(Batch Quality Control)
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Instrument efficiency check
z
z
z
{
Low background check
z
z
z
{
Analyzed daily
Control chart
Establish action limits
Analyzed weekly
Control chart
Establish action limits
Analytical Batch
z
z
z
z
Sample Duplicates at a 10% frequency
Sample Spikes at a 5% frequency
Control chart
Establish action limits
EPA Method 901.1
Interferences
{
Significant interference occurs when
counting a sample with a NaI(Tl)
detector.
z
{
Sample radionuclides emit gamma
photons of nearly identical energies.
Sample homogeneity is important to
gamma count reproducibility and
counting efficiency.
z
Add HNO3 to water sample container to
lessen the problem of radionuclides
adsorbing to the container
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EPA Method 901.1
Application
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The limits set forth in PL 93-523, 40 CFR
34324 recommend that in the case of manmade radionuclides, the limiting
concentration is that which will produce an
annual dose equivalent to 4 mrem/year.
If several radionuclides are present, the
sum of their annual dose equivalent must
not exceed 4 mrem/year.
EPA Method 901.1Calculations
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Gamma radioactivity
Calculations are performed by the instrument
software.
Gamma (pCi/l) =
C
2.22 * BEV
Where:
C= Net count rate, cpm, in the peak area above
baseline continuum
B= the gamma-ray abundance
(gammas/disintegration)
E= detector efficiency (counts/gamma) for the
particular photopeak energy
V= volume of sample aliquot analyzed (liters)
2.22= conversion factor from dpm/pCi
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